1. Field of the Invention
The present invention relates to a single phase motor driving unit.
2. Description of the Related Art
A conventional single phase motor driving unit will be described below with reference to FIGS. 5 and 6. FIG. 5 is a circuit diagram showing the conventional single phase motor driving unit, and FIG. 6 is a waveform chart showing driving waveforms for the conventional single phase motor driving unit.
NPN-type bipolar transistors 2 and 4 (first driving transistors) supply a single phase coil 6 with a driving electric current in the right direction in the drawing when driving signals A and D are supplied. Hence, the collector-emitter path of the bipolar transistor 2, the single phase coil 6, and the collector-emitter path of the bipolar transistor 4 are connected in series between a power source Vcc and a ground Vss. In the same way, NPN-type bipolar transistors 8 and 10 (second driving transistors) supply the single phase coil 6 with a driving electric current in the left direction in the drawing when driving signals C and B are supplied. Hence, the collector-emitter path of the bipolar transistor 8, the single phase coil 6, and the collector-emitter path of the bipolar transistor 10 are connected in series between the power source Vcc and the ground Vss. And by the bipolar transistors 2, 4 and 8, 10 being complementarily switched on and off to change the direction of the driving electric current for the single phase coil 6 as needed, the single phase motor rotates.
Furthermore, the change timings of the driving signals A, B, C, and D are slightly deviated from each other. That is, when the bipolar transistors 2, 4 and 8, 10 are complementarily switched on and off, no electric current passes through between the power source Vcc and the ground Vss and the circuit is not short-circuited because the bipolar transistors 2, 4 and 8, 10 are all off simultaneously for a short time. See Japanese Patent Application Laid-Open Publication No. Hei7-87775.
However, when the bipolar transistors 2, 4 and 8, 10 are complementarily switched on and off, the driving electric current in the right direction in the drawing for the single phase coil 6 circulates along the recirculation path indicated by a broken line to be dissipated in a short time. On the other hand, the driving electric current in the left direction in the drawing for the single phase coil 6 circulates along the recirculation path indicated by an alternate long and short dashed line to be dissipated in a short time. That is, in phase changes where the bipolar transistors 2, 4 and 8, 10 are complementarily switched on and off, the driving electric current for the single phase coil 6 has ineffective electric current portions indicated by oblique lines and does not contribute much to torque, and thus the direction of the driving electric current for the single phase coil 6 suddenly changes. There is a problem that by this, vibrations, noises, and power consumption of the single phase motor become large.